1. Field of the Invention
The present invention relates to a functional substrate represented by an organic EL device, a color filter substrate or the like.
2. Description of the Related Art
An example of the functional substrate is an organic electroluminescent device (hereinafter sometimes referred to as the “organic EL device”). In general, the organic EL device contains an active matrix substrate on which a plurality of pixel electrodes are disposed in the form of a matrix, a partition (bank) for partitioning the plural pixel electrodes, an organic functional layer including an organic electroluminescent luminous layer (hereinafter sometimes referred to as the “organic EL luminous layer”) formed on each of the plural pixel electrodes, and an upper common electrode covering the whole of the organic functional layers and the partition. In the organic EL device, each organic EL luminous layer forms a pixel.
Organic EL devices are roughly classified in accordance with the kind of organic functional layer. Specifically, they are roughly classified into an oligomeric organic EL (OLED) device including an oligomeric organic material as a principal component and a polymeric organic EL (PLED) device including a polymeric organic material as a principal component. In the polymeric organic EL device, the organic functional layer is generally formed by a wet type application method such as an ink-jet method (see, for example, Japanese Laid-Open Patent Publication No. 2000-353594 (Patent Document 1)).
FIG. 6 is a schematic cross-sectional view for explaining a procedure for forming an organic functional layer 120 by the ink-jet method, that is, a kind of the wet type application method.
In general, the organic functional layer 120 is formed as follows: The organic functional layer 120 is formed by dropping, onto an active matrix substrate 110 having a partition 130 thereon, ink droplets including a material for forming the organic functional layer 120 (such as a luminous material) and by thermally drying the resultant.
The drying of the ink droplets is started from a portion in the vicinity of an inclined face (slope) 131 of the partition 130. As shown in FIG. 6, immediately after dropping an ink droplet, the ink droplet is in a shape S5 largely swollen. In accordance with the proceeding of the drying, the shape of the ink droplet is gradually changed from the shape S5 to a shape S6 and from the shape S6 to a shape S7.
In the state of the shape S5 attained immediately after dropping or in the state of the shape S6 obtained when the droplet is slightly dried, a solvent included in the ink droplet is not much dried yet, and hence, the viscosity of the ink droplet is comparatively low. Therefore, at the stage where the ink droplet is dried into the shape S6, an organic material included in the ink droplet is minimally adhered onto the inclined face 131. When the ink droplet is in the shape S7 where the drying is further proceeded, the organic material included in the ink droplet starts to adhere onto the inclined face 131 of the partition 130. This is because the solvent included in the ink droplet is further dried and hence the viscosity of the ink droplet (particularly, a portion thereof disposed in the vicinity of a contact surface with the inclined face of the partition 130) is increased. When the ink droplet is further dried, the viscosity of the ink droplet is gradually increased, and the amount of organic material included in the ink droplet adhering onto the inclined face 131 of the partition is increased, Accordingly, as shown in FIG. 6, an organic functional layer 120 is formed in a concave shape having a larger thickness in a portion thereof close to the partition 130 than a center portion thereof (corresponding to a pixel portion). This phenomenon is generally designated as “pinning”.
When the pinning occurs, the brightness is higher in the center portion of the organic functional layer 120 (namely, a pixel center portion) than in a peripheral portion of the organic functional layer 120 (namely, a pixel peripheral portion), and hence, a brightness spot is caused in each pixel. Accordingly, it is disadvantageously difficult to attain desired display quality.
Also, in the organic functional layer 120, a larger current passes through the center portion having a smaller thickness than through the peripheral portion having a larger thickness. In other words, most of currents passing through the organic functional layer 120 collectively pass through the center portion of the organic functional layer 120. Therefore, the degradation speed of the center portion of the organic functional layer 120 is higher than the degradation speed of the peripheral portion. Accordingly, when the pinning occurs, the life time of the organic EL device is disadvantageously shortened.
Owing to the characteristics of the ink-jet method, it is difficult to increase the concentration of the ink droplet to be dropped beyond a given concentration. Therefore, in order to form an organic functional layer 120 having a large thickness in the center portion, it is necessary to drop an ink droplet with a large volume. In the case where the ink droplet to be dropped has a large volume, it is necessary to provide the partition 130 with a sufficiently large height so that the dropped ink droplet cannot move onto an adjacent pixel electrode beyond the partition. When the partition 130 has a large height, however, the area of the inclined face 131 where the pinning of ink droplets occurs is increased, and hence, the amount of organic material adhering onto the inclined face 131 is increased. Therefore, a difference in the thickness between the center portion and the peripheral portion of the organic functional layer 120 is further increased, and it is impossible to obtain a desired thickness in the center portion of the organic functional layer 120. Therefore, sufficiently high luminous brightness cannot be attained, and hence, there arises a problem that desired image display quality cannot be attained.